1. Field of the Invention
The present invention relates to an electrophotographic toner for negative charging. More particularly, the present invention relates to an electrophotographic toner for negative charging, which is capable of forming a high-density image without scattering of the toner.
2. Description of the Related Art
In commercial electrophotographic reproduction or electrophotographic printing, in order to reduce the amount of ozone generated at the charging step, there is ordinarily adopted a process in which an electrostatic image positively charged is formed, and therefore, a toner for negative charging is widely used as the developing toner for developing this electrostatic image.
Recently, development of a laser beam printer or a digital copying machine has advanced, and in this image-forming apparatus, there is adopted an operation of writing a latent image into an organic photosensitive material of the negative charging type by a laser and performing reversal development by a toner for negative charging, and a high quality is also required for the toner negative charging.
The developing toner is generally formed by pulverizing a resin composition comprising a fixing resin, a colorant and a charge-controlling agent as indispensable components into an average particle size of 5 to 15 .mu.m. Naturally, a charge-controlling agent exerting a negative charge-controlling action at the frictional charging is used in case of a toner for negative charging.
In the conventional toner charge-controlling process, the average value of the charge quantity as the entire toner is controlled according to the kind of the charge-controlling agent or the amount added of the charge-controlling agent. However, even if the average value of the charge quantity as the entire toner can be controlled, it is very difficult to strictly control the distribution of the charge quantity in toner particles.
It is known that a plurality of charge-controlling agents having charging performances reverse to each other are incorporated in toner particles. For example, Japanese Unexamined Pat. Publication No. 54-34243 discloses a developer for developing an electrostatically charged image, comprising a toner and a carrier, in which the toner is a toner for negative charging, which comprises a dye positively charged by friction with the carrier.
Furthermore, Japanese Unexamined Pat. Publication No. 57-196264 discloses an electrically insulating magnetic one-component developer comprising an electrically insulating fixing medium and, dispersed in the fixing medium, a magnetic material powder and a charge-controlling agent, in which the charge-controlling agent comprises a negative or positive charge-controlling agent and a charge-controlling agent having a reverse polarity at a weight ratio of from 1/0.05 to 1/1.5.
In the conventional toner for negative charging, even though the average value of the charge quantity can be maintained at a satisfactory level by adjusting the kind or amount added of the charge-controlling agent, a disadvantage of considerable broadening of the distribution of the charge quantity cannot be eliminated. Namely, a highly charged toner having a much larger charge quantity than the average value, which is not consumed for the development, is inevitably generated at a certain frequency (distribution quantity). Furthermore, a lowly charged toner having a much smaller charge quantity than the average value and causing scattering of the toner is generated at a certain frequency.
Particles of the former highly charged toner are electrically strongly attracted to surfaces of the carrier particles and are present in a hardly separable state, and they extraordinarily inhibit frictional chargeability performances of the carrier particles. Accordingly, even in case of a toner causing no particular problem at the initial stage of the development, with the lapse of the developing time, the proportion of the uncharged or lowly charged toner increases, and such troubles as scattering of the toner, fogging and reduction of the image density are caused.
In the above-mentioned prior art process in which a charge-controlling agent for negative charging is combined with a positively chargeable dye (charge-controlling agent), there can be attained an advantage that the distribution of the charge quantity can be considerably freely shifted to the high charge quantity side or the low charge quantity side, but this process is still insufficient for sharpening the distribution of the charge quantity of the toner and controlling formation of a highly charged toner or a lowly charged toner completely or to a level that can be neglected.